Reformation of conventional natural gas feedstocks create reformate gas having a high concentration of hydrogen, usually with further processing to lower the concentrations of CO, providing hydrogen fuel for fuel cells (and other purposes). The processing typically begins with desulfurization in a catalytic hydrodesulfurizer (HDS). Pipeline natural gas, at times, is injected with concentrations as high as 15 volume percent of olefins (CnH2n) such as ethylene and/or propylene, such as, for instance, to maintain the heating value of the gas.
Pipeline natural gas is often supplemented with peak shave gas, which typically contains propane diluted with air which contains oxygen; sometimes the peak shave gas includes propylene. The olefins and the oxygen react with hydrogen in the HDS, raising the temperature of the catalyst bed to levels which reduce HDS performance and may cause failure of the HDS. In order to protect against such high temperature excursions, a system controller is typically programmed to respond to excess temperature by reducing the power output of the fuel cell or other reformate consuming system (sometimes referred to as “foldback”), and if the problem is not thereby corrected, by shutting down the fuel cell or other reformate consuming system.
Olefins react over the hydrodesulfurizer catalyst in the presence of hydrogen to form alkanes (CnH2n+2), generating heat. For example, ethylene (reaction 1) and propylene (reaction 2) react to form ethane and propane:C2H4+H2→C2H6+heat  (1)C3H6+H2→C3H8+heat  (2)
The oxygen and any propylene in the peak shave gas will also react over the hydrodesulfurizer catalyst in the presence of hydrogen to form water and propane, generating heat. Oxygen and hydrogen (reaction 3) forms water, and propylene reacts with hydrogen (reaction 2) to form propane.H2+½O2→H2O+heat  (3)These reactions are highly exothermic, for instance, generating temperature rises of up to 28 C (50 F) per percent of olefin or oxygen concentration in the feedstock, which may be as high as 15%.